1. Field of Invention
This invention relates to a probe driving mechanism for displacement measuring apparatuses, and more particularly to an improvement in a probe driving mechanism for displacement measuring apparatuses, suitably used for the measurement of a soft workpiece, such as a rubber workpiece and a spring, and adapted to measure the size of a workpiece without causing the workpiece to be deformed even when a probe is brought into contact therewith.
2. Description of Related Art
The known displacement measuring apparatuses having a comparatively large range of measurement and capable of simply measuring the size of a workpiece include displacement measuring apparatuses of the type which are adapted to move a probe by a motor. Such devices bring the probe into contact with a surface of a workpiece under a predetermined pressure and detect a quantity of movement of the probe as disclosed in, for example, Japanese patent JP-B-58-10681 and Japanese patent JP-B-59-19283.
The applicant of the present invention proposed in Japanese patent JP-B-7-78403, as a displacement measuring apparatus obtained by making improvements on such displacement measuring apparatuses, and capable of conducting the measurement of the size of a workpiece under a uniform and very low measuring pressure with a high accuracy. The displacement measuring apparatus is formed by providing a parallel link mechanism on a slider capable of being moved reciprocatingly in accordance with an operation of a motor. A probe is provided on one end portion of this parallel link mechanism and a weight on the other end which is capable of applying a predetermined pressure to a workpiece via the probe owing to the weight balance between a probe-side portion of the slider and a weight-side portion thereof.
Since a rotational frequency of a motor may be detected by utilizing its back electromotive force, it is necessary that an amplification degree of a detecting circuit during a control operation carried out when the motor is operated at a low speed at which a back electromotive force is small, be set high. Therefore, it is difficult to carry out volume regulation for executing, for example, a pulse-width control operation in accordance with a detected back electromotive force, and the apparatus lacks stability.
Although it is conceivable that detectors, such as a rotary encoder and a tachometer generator be provided additionally without using a back electromotive force of the motor, the necessity of additionally providing these new detectors causes the cost of manufacturing the apparatus and bulk thereof to increase.
Furthermore, since the operation of the probe is stopped basically when the probe contacts the workpiece, it is impossible that the controlling of the motor be done until the mentioned weight balance is attained, by only an output from the displacement detector of a position measuring scale (having another name of a linear encoder including a main scale and a displacement detector).